Interruption of the blood supply to neural tissues, such as the brain, can cause a complex series of biochemical changes which may result in neuronal cell damage. At the cellular level, it is generally understood that damage is mediated by opening of the N-methyl-D-aspartate (NMDA) channels in the membrane. Further, ischemia begins when the blood supply stops or is significantly slowed, and this ischemia phase may be followed by restoration of the blood supply during a reperfusion phase. It is understood that cellular damage may occur during both phases, though they occur through different mechanisms.
There are a complex series of events which contribute to cell death during ischemia/reperfusion. Six substances that accumulate during ischemia include excitatory amino acids, intracellular calcium, arachidonic and other free fatty acids, hypoxanthine, xanthine oxidase, and platelet activating factor.
Ischemia triggers at least three pathways deleterious to the cell. First, a lack of oxygen depletes energy stores (principally adenosine diphosphate known as ATP). This disrupts homeostatic mechanisms, most importantly the membrane pump mechanism that maintains intracellular calcium at low levels. The resulting rise in intracellular calcium, which occurs principally because of the opening of the N-methyl-D-aspartate (NMDA) channels in the membrane, increases release of glutamic acid, activates destructive proteases and lipases, and indirectly converts the enzyme xanthine dehydrogenase to the potentially harmful xanthine oxidase. Second, excitatory amino acids (“excitotoxins”), principally glutamic and aspartic acids, are released, activate calcium channels, further increase intracellular calcium through a positive feedback mechanism, and allow cellular entry of excess water, sodium and chloride. Third, acidosis enhances destructive lipid peroxidation and the release of damaging free radicals.
Upon restoration of the blood supply, the reperfusion phase begins. An increased intracellular calcium level, a result of opened NMDA channels during ischemia, triggers a more destructive cascade. The initial calcium impulse causes a cascade which results in the release of intracellular calcium stores from the intravesicular calcium deposit. The release of intracellular calcium is mediated via the ryanodine receptor, principally the type 3 ryanodine receptor. The net result is a thirtyfold rise in intracellular calcium and cell death. Attempts have been made to reperfuse as soon as possible after the onset of ischemia, but it is important to note that the reperfusion itself causes the cascade, therefore the neurodestructive phases of ischemia and reperfusion are distinct.
Neurophysiologists view reperfusion injury as a cascade process that leads to excitotoxic cell death. The rise in intracellular calcium during reperfusion causes vasoconstriction of neighboring blood vessels. In addition, it causes the release of free oxygen radicals, in part from the action of xanthine oxidase. The net result is excitotoxic neuronal cell death.
Increased cytosolic Ca2+ concentration contributes significantly to neuronal cell damage during ischemic reperfusion. U.S. Pat. No. 6,462,066 to Mangat et al. (which is incorporated herein as if fully rewritten) describes the above phenomena of ischemic injury and discusses the use of dantrolene to prevent or minimize neuronal cell damage that occurs during the reperfusion phase of an ischemic episode.
Dantrolene is an antagonist of the type 3 ryanodine receptor and is commonly given as the sodium salt (sodium dantrium), which is hydrated 1-[[[5-(4-nitrophenyl)2-furanyl]methylene]amino]-2,4-imidazolidinedione sodium salt. Dantrolene is prescribed in the treatment of clinical spasticity resulting from upper motor neuron disorders such as spinal chord injury, cerebral palsy, stroke, or multiple sclerosis. Dantrolene is also effective in reversing the hypermetabolic process of malignant hyperthermia, a genetic disorder of skeletal muscle that is triggered by exposure to anesthetics and certain relaxants.
The conflict in Iraq has produced an unprecedented number of traumatic brain injuries and has radically changed the way we treat trauma with the advent of Combat Surgical Hospitals on the frontline with injured troops arriving within an hour of injury. A patient might remain in the combat hospital for only six hours. The goal is lightning-swift, expert treatment, followed as quickly as possible by transfer to the military hospital in Landstuhl, Germany, for continued treatment.
American troops injured in Iraq have required limb amputations at twice the rate of past wars, and as many as 20 percent have suffered head and neck injuries that may require a lifetime of care. Accurate statistics are not yet available on recovery from this new round of battlefield brain (traumatic brain) injuries, an obstacle that frustrates combat surgeons. But judging by medical literature and surgeons' experience with their own patients some experts believe that three or four months from injury, 50 to 60 percent will be functional and doing things. In other words, these patients may be up and around, but with pretty significant disabilities, including paralysis. The remaining 40 percent to 50 percent of patients include those whom the surgeons send to Europe, and on to the United States, may have no prospect of regaining consciousness.
Preventing or minimizing neuronal cell damage that occurs during the reperfusion phase of an ischemic episode by virtue of a combat injury or other traumatic events which cause brain injuries or contusions with a non invasive administration of compounds which achieve higher and faster CNS penetration than dantrolene would be highly desirable and life saving.
Severe cerebral contusion is sometimes associated with early edema formation within 24-48 hours post-trauma, and this frequently results in progressive ICP (intracranial pressure) elevation, clinical deterioration and swelling. This swelling causes pressure on the brain squeezing it in the cranium (skull) and causing ischemic changes to the brain, often occluding fine blood supply to critical areas of the brain. Once steroids are on board to shrink the swelling there will be a secondary reperfusion injury as blood supply is re-established which contributes to further edema and neuronal cell death.
The mechanism of cell death for neurons is via the release of intracellular calcium. This leads to neuronal cell swelling/death and edema. This pathway occurs both at initial ischemic insult and when the reperfusion injury occurs.
In another aspect, it has been recognized that certain nerve gasses based upon organophosphorus compounds, such as sarin, soman, tabun and cylcosarin (cyclohexyl methylphosphonofluoridate, a gas known as GF) cause ischemic injuries by generally the same mechanism as severe cerebral contusions. It has been found that the method of administration of dantrolene has not been successful in achieving effective neuroprotection against nerve gas attack.